Hartke kickback 10 vs 12 vs 15

Does anyone have any input about the difference in sound quality for the three different sizes of hartke kickback combo amps. I'm thinking about buying either the 10 or the 15 but I've heard that 15 inch speakers generally sound like crap for upright bass. Is this true? Are there any other decent combo amps within a similar price range ( 300-600) that sound good for double bass.